1. Field of the Invention
The present invention relates to devices and methods for controlling a limited slip differential capable of limiting the differential operation of front and rear wheels of a four-wheel-drive vehicle.
2. Description of the Related Art
Conventionally, four-wheel-drive vehicles are known in the art which include a limited slip differential capable of limiting the differential operation of front and rear wheels of the four-wheel-drive vehicle and a vehicle behavior control device that controls behavior of the vehicle by applying a braking force.
For example, a four-wheel-drive vehicle described in paragraph [0073] of Japanese Patent Application Publication No. 2003-231428 (JP 2003-231428 A) includes a driving force distribution controller that controls distribution of a driving force to front and rear wheels, and a stability controller that controls behavior of the vehicle by applying a braking force. The driving force distribution controller controls distribution of the driving force to the front and rear wheels by controlling an engaging force of a clutch inserted in a driving force transmission path from an engine serving as a driving source to the front wheels serving as driven wheels.
If the engaging force of the clutch is increased by the control of the driving force distribution controller, the four-wheel-drive vehicle is switched to a four-wheel-drive state where a differential operation of the front and rear wheels is limited. However, if this clutch is disengaged, the differential operation of the front and rear wheels is no longer limited, and the four-wheel-drive vehicle is switched to a two-wheel-drive state where the driving force of the driving source is transmitted only to the rear wheels serving as main driving wheels. For example, if differential rotation between the front and rear wheels increases due to slipping of the front or rear wheels, the driving force distribution controller increases the engaging force of the clutch to switch the drive state to the four-wheel-drive state, thereby suppressing the slipping.
When the stability controller is performing vehicle behavior stabilization control, the driving force distribution controller limits the engaging force of the clutch according to the magnitude of the applied braking force. This suppresses interference between the control by the driving force distribution controller and the control by the stability controller.
For example, when a vehicle is traveling on a low μ road such as a gravel road, the differential rotation between the front and rear wheels tends to increase, and the drive state is more frequently switched to the four-wheel-drive state. If such vehicle behavior stabilization control is performed when the four-wheel-drive vehicle described in JP 2003-231428 A is in the four-wheel-drive state, the engaging force of the clutch is limited according to the braking force. Accordingly, frictional members of the clutch may frictionally slide on each other, and frictional heat may be generated by this frictional sliding. If a thermal load increases due to the heat generated in the clutch, the clutch may be kept in a disengaged state (two-wheel-drive state) until the temperature of the clutch decreases to a predetermined value or lower, in order to protect the clutch from being damaged by overheat.
In such a case, even if differential rotation between the front and rear wheels increases due to slipping of the main driving wheels, the drive state cannot be switched to the four-wheel-drive state, which reduces traveling stability on a low μ road.